The Fuel Property Characterization (FPC) effort within the Co-Optimization of Fuels and Engines (Co-Optima) initiative focuses on the measurement of critical fuel properties. The goal of Co-Optima is to leverage unique fuel chemistries available from biomass to design more efficient engines, thereby reducing energy consumption and environmental impacts of transportation. FPC measurement of critical fuel properties supports the on-going efforts of blendstock generation, structure-property relationships, and analysis, through multiple channels. FPC is responsible for the development, expansion, and, maintenance of the fuel property database (FPD) - including acquisition of fuel property data. The FPD was heavily utilized during tiered screening approaches to rapidly identify the most promising blendstocks for Boosted Spark Ignition (BSI) and Mixing-Controlled Compression Ignition (MCCI) combustion. A key outcome of this endeavor is that it directly led to the identification and more in-depth evaluation of ten of the most promising blendstocks for BSI as well as the initial selection of 12 MCCI candidates for further consideration. Future efforts will also rely on the FPD as a screening tool as additional combustion approaches are investigated. FPC supports analysis efforts through the performance of compatibility and toxicology assessment of promising candidates. Through fundamental experimental measurements in a flow reactor, FPC provides critical feedback to the mechanistic understanding of soot precursor formation and the validation of kinetic mechanisms. These experiments combined with quantum mechanical calculations showed mechanistically why different functional group location could lead to widely varying soot production - even for very similar molecules such as positional isomers. Additionally, experiments are beginning to reveal the chemical basis for non-linear blending effects for octane number which will allow the design of molecules with desired blending octane behavior in future endeavors. The outcomes of these efforts will help ensure that the Co-Optima program can identify fuel-engine combinations which achieve Co-Optima's efficiency, environmental and economic goals.